If following the original runway's GA leads to a hazard (as at LHR) then you will cause considerable surprise in the tower following a procedure that is not expected by ATC
At Heathrow, if you are on the ILS 27R, and we switch you visually to 27L, we will expect you to carry out the
27R missed approach procedure (i.e. the one you briefed, and the one you set up for).
Thus, you can be sure we will give you positive missed approach instructions ASAP.